Strontium isopropoxide: a highly active catalyst for the ring‐opening polymerization of lactide and various lactones

Abstract Commercially available strontium isopropoxide represents a suitable catalyst/initiator for the ring‐opening polymerization (ROP) of lactide (LA), ε−caprolactone, δ−valerolactone, δ−caprolactone, and δ−decalactone. Well‐defined polyesters are accessible via the solution polymerization of lactide in toluene with a [LA]:[Sr] ratio of 100:1 at room temperature with or without the addition of dodecanol as coinitiator. Kinetic studies and detailed analysis by means of matrix‐assisted laser desorption ionization mass spectrometry reveal pseudo‐first‐order kinetics of the ROP as well as excellent endgroup fidelity of the polylactide (PLA) with isopropyl and dodecyl α‐endgroups. Both isopropanolate moieties as well as the coinitiator each initiate PLA chains, enabling the synthesis of PLA with tailored molar mass. The polymerization of ε−caprolactone and δ−valerolactone confirms the high catalyst activity, which causes quantitative monomer conversion after 1 min polymerization time but broad molar mass distributions. In contrast, the catalyst is well suited for the ROP of the less reactive δ−caprolactone and δ−decalactone. Although kinetic studies reveal initially bimodal molar mass distributions, polyesters with dispersity values Ð < 1.2 and unimodal molar mass distributions can be obtained at moderate to high monomer conversions.